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Carbohydrate Polymers 82 (2010) 561–568

Contents lists available at ScienceDirect

Carbohydrate Polymers

journa l homepage: www.e lsev ier .com/ locate /carbpol

odified cassava starches as corrosion inhibitors of carbon steel:n electrochemical and morphological approach

arisela Belloa, Nathalie Ochoaa,∗, Vittoria Balsamoa, Francisco López-Carrasquerob,antiago Coll c, Antonio Monsalved, Gema Gonzálezd

Departamento de Ciencia de los Materiales, Universidad Simón Bolívar, Aptdo. 89000, Caracas, VenezuelaGrupo de Polímeros ULA, Departamento de Química, Facultad de Ciencias, Universidad de los Andes, Mérida 5101A, VenezuelaLaboratorio de Química Computacional, Centro de Química, Instituto Venezolano de Investigaciones Científicas (IVIC), Altos de Pipe,arretera Panamericana Km. 11, Edo. Miranda, VenezuelaLaboratorio de Materiales, Centro de Ingeniería de Materiales y Nanotecnología, Instituto Venezolano de Investigaciones Científicas (IVIC), Altos de Pipe,arretera Panamericana Km. 11, Edo. Miranda, Venezuela

r t i c l e i n f o

rticle history:eceived 23 March 2010eceived in revised form 6 May 2010ccepted 7 May 2010vailable online 21 May 2010

eywords:

a b s t r a c t

The physical and chemical modification of cassava starch was carried out to prepare compounds thatwere evaluated as corrosion inhibitors of carbon steel under alkaline conditions in 200 mg L−1 NaCl solu-tions. Two species were tested: an activated starch (AS) and a carboxymethylated starch (CMS) withtwo different degrees of substitution (DS). The success of the chemical procedure was confirmed by 13CNMR. The DS was determined by back titration and products with 0.13 (CMS0.13) and 0.24 (CMS0.24) wereobtained. The inhibitive properties were studied by means of electrochemical impedance spectroscopy.

assava starchhysical–chemical modificationorrosion inhibitionarbon steel

It was found that modified starches have corrosion inhibitive properties and that their protection leveldepends on the type and amount of active groups present in the molecules. AS showed better performancethan CMS, whose inhibition ability increases with the degree of substitution. This result was explainedby the strong ionic interaction between AS and ferrous cations, which was confirmed by the electro-static potential mapping of the monomeric units. After the corrosion experiments, the surfaces also were

mic figher

investigated through atowas responsible for the h

. Introduction

Corrosion problems are the main degradation cause of cool-ng water systems made of carbon steel. This phenomenon entailsconomic costs due to, for example, metallic material loses andacteria development. In order to prevent degradation of these sys-ems, water is frequently treated with formulations composed oforrosion inhibitors and biocides (Frenier, 2000).

Studies on inhibitors have evolved since 1950; at the begin-ing, the investigations were focused on the efficiency evaluationf chromates, nitrates, and borates, which showed high effi-

iency, but also high toxicity for the human being (Kàlmàn,990). Other compounds, economically profitable, such as metallications, polyphosphates, gluconates, vanadates, molybdates, phos-honates, polyacrylates, and carboxylates have also been evaluated

∗ Corresponding author at: Universidad Simón Bolívar, Departamento de Cienciae los Materiales, Carretera de Hoyo de la Puerta, Valle de Sartenejas Municipioaruta, Apartado 89000, Caracas 1080-A, Venezuela. Tel.: +58 212 9063945;

ax: +58 212 9063932.E-mail address: nochoa@usb.ve (N. Ochoa).

144-8617/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.carbpol.2010.05.019

orce microscopy. It was found that a densification of the inhibitive layerprotection level afforded by AS after 24 h of immersion.

© 2010 Elsevier Ltd. All rights reserved.

(Frenier, 2000). However, these compounds have been banned inenvironmental regulations because they exhibit low biodegrad-ability and are responsible for the eutrophication phenomenon(Fiaud, Pébère, & Zucchi, 2002). From the late 80s, most corro-sion inhibitors studies have been focused on the developmentof “environmental friendly” compounds in response to legislationchanges concerning environmental protection. This term includesformulations that are not toxic to humans, have low environmen-tal impact, optimal biodegradability, and maintain their efficiencyand cost-effectiveness. Thus, since the 90s, many investigationshave been related to the evaluation of natural compounds ascorrosion inhibitors; in this sense, some amino acids, vitamins,plant extracts, and soluble natural polymers have been tested(Abd El Haleem, Abd El Rehim, & Shalaby, 1986; Bouyanzer, 2004;Bouyanzer, Hammouti, & Majidi, 2006; Chauhan & Gunasekaran,2007; El-Etre, 2006; El-Etre, 2007; El-Etre, 2008; Gunasekaran &Chauhan, 2004; Ismail, 2007; Oguzie, 2001; Radojcic, Berkovic,

Kovac, & Vorkapic-Furac, 2008; Rahim et al., 2007; Sugama &DuVall, 1996). Nevertheless, few investigations report the behav-ior of these compounds in neutral/alkaline environments (AbdEl Haleem et al., 1986; Bouyanzer, 2004; Bouyanzer et al., 2006;Chauhan & Gunasekaran, 2007; El-Etre, 2006; El-Etre, 2007; El-Etre,

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62 M. Bello et al. / Carbohydra

008; Frenier, 2000; Ismail, 2007; Oguzie, 2001; Rahim et al., 2007;ugama & DuVall, 1996) in comparison with those carried out incid medium. It is therefore the aim of this study is to evaluateassava starches as corrosion inhibitors of carbon steel in alkalineodium chloride solution.

Starch is a natural polymer, highly available at low cost,enewable, and biodegradable. It is widely used in differentndustries such as pharmaceutical, papermaking, textile, and inood preparation. However, native starch presents some draw-acks such as low water solubility and poor stability when it issed at low pH (Burrell, 2003). Physical and/or chemical mod-

fications are therefore interesting alternatives to develop newroducts, preserving petrochemical resources and improving spe-ific starch properties (Balsamo et al., 2010; Heinze & Koschella,005; Rivero, Balsamo, & Müller, 2009; Singh, Kaur, & McCarthy,007). In this work two types of modified cassava starches wererepared in order to be studied as corrosion inhibitors: AS, aassava starch that was modified through gelatinization and acti-ation and CMS (carboxymethylated starch) with two differentegrees of substitution (DS). These species were tested as cor-osion inhibitors of carbon steel in an alkaline 200 mg L−1 NaClolution that is a simplification of the chemical composition of tapater.

It is important to point out that, up to our knowledge, feworks have been reported in the literature in which starch has

een used to protect metals against corrosion. Sugama and DuVall1996) reported the use of potato starch coatings for aluminum.hey found that these coating films displayed a low susceptibilityo moisture and improved impedance (in � cm2) by two orders of

agnitude over that of the substrate. Abd El Haleem et al. (1986)ave evaluated a starch among several natural compounds to pro-ide protection to pitting corrosion of steel; nevertheless, they didot specify the kind of starch they employed. More recently, Roslizand Wan Nik reported in 2009 the use of cassava starch to improvehe corrosion resistance of an aluminum alloy in seawater. Theyound that the introduction of native tapioca starch minimized theeight loss and abridged aluminum dissolution in seawater.

. Experimental

.1. Preparation of modified cassava starches

Corrosion inhibitors, AS and CMS, were prepared from nativeassava starch (30% amylose), supplied by Agroindustriales Man-ioca S.A., Venezuela, following the procedure described by Feijoot al. 50 g of native starch was placed in a glass reactor with dis-illed water (10%, w/w) and 250 ml of a 10% (w/v) NaOH aqueousolution was added. The slurry was heated up to 60 ◦C and keptt that temperature under mechanical agitation for 2 h in ordero activate the starch molecules. A fraction of this mixture wasrecipitated and washed by adding ethanol in order to obtain thectivated starch (AS). Then, the temperature of the remaining frac-ion was reduced down to 50 ◦C and an aqueous solution (10%, w/v)f monochloroacetic acid (MCAA) was added in order to synthesizehe carboxymethylated starch (CMS); for this chemical modifica-ion a molar ratio NaOH:AGU:MCAA of 2:1:1 was used, where AGUefers to the starch anhydroglucose unit. The carboxymethylationrocess was carried out for 3 h under mechanical agitation. Thenal product, CMS, was precipitated and washed by adding ethanol.oth products, AS and CMS, called from now on “inhibitors”, were

yophilized for 48 h, and stored under vacuum at 4 ◦C.

.2. Characterization of the inhibitors

The inhibitors were characterized using Nuclear Magnetic Res-nance. 13C NMR spectra of AS and CMS were collected on a

ymers 82 (2010) 561–568

Bruker AVANCE-500, 125.75 MHz, at 70 ◦C, using DMSO-d6 andD2O, respectively. For this purpose, the samples were previouslydissolved at room temperature and kept under agitation for 24 h.

The degree of substitution of CMS, DS, defined as the averageof hydroxyl groups substituted per AGU unit, was determined byan acid–base back titration (Contó, 2008; Heinze, Pfeiffer, Liebert, &Heinze, 1999; Stojanovi, Jeremi, Jovanovi, & Lechner, 2005). For thispurpose, the CMS was dispersed in acetone under magnetic stir-ring and transformed into its protonated form (H-CMS) by addinga 6 M HCl aqueous solution and stirring for 30 min. The precipitatewas re-dispersed in acetone, filtered, and lyophilized for 24 h. Then,25 ml of a 0.04 M NaOH solution and 75 ml of distilled water wereadded to 0.1250 g (±0.0001) of CMS and agitated during 4 h beforetitration. The excess of alkali was back titrated with a standard0.05 M HCl solution using phenolphthalein as indicator. A nativestarch sample was titrated as a blank reference. The DS was deter-mined using the following equation:

DS = MWAGU(Vblank − Vsample)NW − 58(Vblank − Vsample)N

(1)

where MWAGU is the molar weight of the anhydroglucose unit(162 g/mol), Vblank and Vsample are the HCl volumes used for titra-tion of the blank and sample, respectively, W is the weight of thesample (g), and N is the normality of the HCl solution.

2.3. Theoretical details

Electrostatic potential [V(r)] mapping of the repetitive unitof AS and CMS was studied in order to explore the nature ofactive sites. These calculations are based on the model proposedby Politzer and Sjoberg, who have shown that by computingV(r) on the 0.002-electron/bohr3 contour isosurface (Sjoberg &Politzer, 1990) of the molecular electronic density �(r), it couldquantify the susceptibility of molecules to electrophilic and/ornucleophilic attack. In other words, this methodology allows deter-mining Lewis acid and basic sites in a molecule (Aray et al., 2003;Aray, Rodríguez, Coll, González, & Márquez, 2004; Gadre & Shirsat,2000; Leboeuf, Koster, Jug, & Salahub, 1999; Murray & Sen, 1996;Orozco & Luque, 1996; Politzer & Truhlar, 1982), which corre-spond to electron-poor and electron-rich zones of the repetitiveunit, respectively. Additionally, the active sites’ susceptibility canbe quantified determining the minimum and maximum V(r) valuesat the determined host zones using a Newton–Raphson techniquesimilar to those that have been previously reported by Aray etal. for the study of the electronic density and electrostatic poten-tial topology (Aray et al., 2003; Aray et al., 2004; Aray, Rodríguez,Coll, Rodríguez-Arias, & Vega, 2005). DMol3 program (Delley, 1990;Delley, 2000) was used to calculate the molecular electronic density(�(r)) and V(r). DMol3 calculates variational self-consistent solu-tions to the density functional theory (DFT) equations, expressedin an accurate numerical atomic orbital basis. The solutions tothese equations provide the molecular electron densities, whichcan be used to evaluate the total electrostatic potential of thesystem. The correlation and exchange effects were consideredusing the gradient generalized approximation (GGA) and the func-tional Perdew-Wang 91 (Perdew & Wang, 1992). The numericaldouble-zeta plus polarization basis set (DNP) was used in all calcu-lations.

2.4. Corrosion experiments

The inhibitive properties of modified cassava starches wereevaluated using electrochemical impedance spectroscopy (EIS).This technique was coupled with a rotating disk electrode witha fixed rotation speed of 1000 rpm. Impedance diagrams were

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of the carboxymethyl moieties results in a new peak at ∼70.6 ppmdue to the C7 methylene group (Lazik, Heinze, & Pfeiffer, 2002).Nevertheless, it should be mentioned that this signal might be over-lapped by the terminal C4 groups of starch. For this reason, we

M. Bello et al. / Carbohydra

erformed using a Gamry Potentiostat DC-105 model. Elec-rochemical impedance measurements were obtained at theorrosion potential. They were carried out under potentiostaticegulation in a frequency range of 100 kHz to 6 mHz withight points per decade using a 10 mV peak-to-peak sinusoidalotential.

The corrosive medium was a 200 mg L−1 sodium chlorideolution in contact with air, maintained at 25 ◦C. This medium rep-esents a simplified sample of the chemical composition of naturalaters, often employed by other authors (Duprat, Lafont, Dabosi, &oran, 1985; Ochoa, Moran, & Pébère, 2004; Ochoa, Moran, Pébère,Tribollet, 2005). The inhibitive solutions were prepared by adding

00 mg of modified starches, AS or CMS, to 150 mL of the corrosiveolution and heating at 30 ◦C under magnetic stirring during 5 min.hen, this solution was transferred to a larger flask to complete 1iter with the remaining corrosive solution. Finally, this 600 mg L−1

olution was allowed to cool down to room temperature under agi-ation. The pH of the solutions was modified by adding NaOH inrder to study the inhibitive properties of the starch solutions atH=10.

Corrosion experiments were performed in a standard Pyrex®ouble wall cell with platinum grid and a saturated calomel elec-rode (SCE) as a counter-electrode and reference, respectively. Theorking electrode was a rod of XC 35 carbon steel having a cross-

ectional area of 0.79 cm2, whose nominal chemical compositions 0.35C, 0.65Mn, 0.25Si, 0.035P, 0.035S, and Fe balance. The elec-rode surface was covered by a heat-shrinkable sheath to leave onlyhe cross-sectional area of the carbon steel cylinder in contact withhe corrosive solution. Before electrochemical tests, steel samplesere polished with SiC paper down to grade 1200. They also were

leaned with ethanol, rinsed with distilled water, and dried witharm air.

.5. Surface analysis

Once the corrosion tests were carried out, photographs ofhe electrodes surfaces were taken with a stereoscopic magni-er. The inhibitive layer formed in the presence of the starcholecules was also analyzed by means of atomic force microscopy

AFM) with an Agilent 5500 Scanning Probe Microscope. AFMmages were obtained using Acoustic AC Mode (AAC Mode) with

cantilever oscillation frequency of 155 kHz. For this purpose,arbon steel samples were immersed during 6 and 24 h in theolution containing the inhibitor. Images were taken at the cen-er and periphery of the surface and processed with Picoscanmage 5 software obtaining topography, amplitude, and phasemages.

. Results and discussion

.1. Structural characterization of modified starches

In a previous work, Feijoo et al. investigated different param-ters in order to optimize the reaction conditions for producingarboxymethylated starch (CMS). Based on their work, we selectedhe conditions indicated in the experimental part to obtain a goodalance between the degree of substitution (DS) and the moleculareight. It must be remarked that under these reaction condi-

ions starch destructurization or gelatinization takes place. Thehemical changes of the starch were verified by 13C NMR spec-roscopy. The 13C NMR spectrum of activated starch is shown in

ig. 1. It exhibits the characteristic peaks of the anhydroglucosenit (Atichokudomchai & Varavinit, 2004; Chi & Txu, 2008; Wang,u, & Yu, 2008) at 78.78, 73.30, 71.98, and 71.64 ppm, which cor-espond to C4, C5, C3, and C2. Other two signals are observed at00.85 and 100.27 due to C1 (1–6) and C1 (1–4), i.e., to C1 in amy-

Fig. 1. 13C NMR spectrum of AS in DMSO-d6 at 70 ◦C.

lose and amylopectine, respectively, while the signals of C6 (1–6)and C6 (1–4) appear at 60.89 and 60.54 ppm. In Fig. 2a, it is shownthe 13C NMR spectrum for one of the CMS samples; the introduction

Fig. 2. (a) 13C NMR and (b) DEPT 135 spectra of CMS0.24 in D2O at 70 ◦C. R = Na+ orCH2COO−Na+.

564 M. Bello et al. / Carbohydrate Polymers 82 (2010) 561–568

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n the 200 mg L−1 NaCl solution and in the presence of 600 mg L−1 of each inhibitor.

erformed DEPT 135 experiments (Fig. 2b); the negative peaks ofhe methylene groups C7 may now be unambiguously identifiedt 70.30 and 71.70 ppm when the substitution has taken place on-6 and O-2. The carboxymethylation on O-2 (C-1s) may also be

dentified in Fig. 2 by the appearance of a signal at ∼97.6 ppm. Inddition, the peak at 178 ppm, which corresponds to the carbonyl8 group, evidences the modification on O-6. The transformation ofhe carboxylate groups into carboxylic groups allowed us the esti-

ation of a degree of substitution (DS), which was of 0.13 ± 0.03CMS0.13) and 0.24 ± 0.04 (CMS0.24) for the two samples that wererepared.

.2. Evaluation of the corrosion inhibitive properties

Fig. 3 shows the electrochemical impedance diagrams recordedt the corrosion potential after 2 h of immersion in solutions con-aining 600 mg L−1 of AS, CMS0.13, and CMS0.24. They are comparedith the diagram obtained for the blank solution. The corrosionotential and the polarization resistance values (Rp) obtained fromhese diagrams are listed in Table 1. Rp values were obtained byxtrapolating the impedance spectra on the real axis at the lowerrequency domain and these values are inversely proportional tohe electrode corrosion rate. It may be observed that the diagramsxhibit one capacitive loop whose size increases in the presence ofnhibitors. This is in agreement with the significant increase of thep values presented in Table 1 when starch molecules are addedo the corrosive solution. In addition, it should be remarked thatn the presence of these species, the polarization resistance valuesncrease in the following order CMS0.13 < CMS0.24 < AS. This results in agreement with the electrode surfaces shown in Fig. 4, wheret can be seen that corrosion spots decrease by increasing the CMSegree of substitution, while the metal surface remains bright whenS was employed. This evidences that starch molecules act as cor-osion inhibitors of carbon steel and the protection level that theyrovide depends on the amount and type of active group present

n the biopolymer: carboxylate (–COO−) and alkoxy (–CO−) groups−

or CMS, and alkoxy (–CO ) groups for AS.

In previous works (Sancristobal, 2009), it was demonstrated by-ray photoelectron spectroscopy (XPS), that the inhibitive film isomposed by an iron oxide–hydroxide layer (Fe2O3 and Fe(OH)3),n which starch molecules are incorporated. Thus, protection pro-

able 1lectrochemical parameters obtained from the impedance diagrams presented inig. 3.

Solution Ecorr vs SCE (mV) Rp (� cm2)

Blank −420 ± 7 1650 ± 307CMS0.13 −432 ± 10 11,600 ± 989CMS0.24 −425 ± 5 20,000 ± 1000AS −399 ± 15 57,450 ± 1023

Fig. 4. Photographs of the XC 35 carbon steel surfaces after 2 h of immersion withoutinhibitor and in the presence of 600 mg L−1 of each inhibitor.

vided by these modified starches is related to the formation ofan insoluble chelate between the active groups present in theinhibitor molecules and the ferrous cations generated when cor-rosion takes place. In agreement with other reports found in theliterature (Ochoa et al., 2004; Suzuki, Nishihara, & Aramaki, 1996),this insoluble complex would seal the defects of a very thin layerof corrosion products, reducing metal dissolution.

Electrostatic potential mappings of AS and CMS monomericunits were carried out in order to explain the differences in theprotection level achieved in the presence of each starch molecule

and they are presented in Fig. 5. Calculations were carried outfrom the salt form units (Na-CMS and Na-AS) removing only onesodium cation from the carboxylate and alkoxy groups, respec-tively. Mapping on this isosurface the V(r) values onto colors allows

M. Bello et al. / Carbohydrate Polymers 82 (2010) 561–568 565

Fig. 5. Electrostatic potential mapping of the CMS and AS monomeric unit. White,red, gray and black spheres correspond to H, O, C, and Na atoms, respectively. Smalllbr

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ight green spheres localize the maxima (at the acid zones) and the minima (at theasic zones). (For interpretation of the references to color in this figure legend, theeader is referred to the web version of the article.)

dentifying the host sites in which nucleophiles (most positiveone) and electrophiles (most negative zone) should bind. As its illustrated in the figure, positive and negative zones are repre-ented in red and blue colors, respectively. Thus, blue zones areocated around –COO− and –CO− species; these zones would inter-ct with the metal surface forming an insoluble chelate accordingo the inhibition mechanism proposed above. As it may be seen,S has a more negative electrostatic potential than CMS; this sug-ests that AS would strongly interact with the metallic surface. Thisesult combined with the lower hydrophilicity (Heinze & Koschella,005) and higher molecular weight (Contó, 2008) of AS explainhe higher polarization resistance values obtained in the presencef this compound. On the other hand, the lower protection levelchieved in the presence of CMS0.13 is related to its lower amountf active groups compared to those available in CMS0.24 (Bello,choa, Balsamo, & Sancristobal, 2010). Indeed, the degree of substi-

ution indicates that the amount of carboxylate groups decreasedn 50%.

Since electrochemical results reveal that the protection levelchieved by AS is higher than for CMS, inhibitive properties of thisolecule were monitored during 24 h of immersion by impedance

Fig. 6. Electrochemical impedance diagrams obtained after 2, 6 and 24 h of immer-sion at Ecorr in the solution containing 600 mg L−1 of AS.

measurements. As shown in Fig. 6, it may be noticed that the sizeof the impedance diagrams shows a significant increase between2 and 6 h of immersion and then it remains constant. In addition,from 6 h, a capacitive loop appears in the high frequency domain,which may be attributed to the development of a thin film thatcovers the metal surface as a paint coating (Duval, Keddam, Sfaira,Srhiri, & Takenouiti, 2002; Gabrielli, 1984). This result reveals thatthe protective properties of AS enhance with the immersion time;in fact, the electrode photographs presented in Figs. 7 and 8 showthat the metal surface remains bright and that there are not tracesof rust after 6 and 24 h of immersion.

Inhibitive films formed in the presence of 600 mg L−1 of AS after6 and 24 h immersion were analyzed by AFM in order to study themorphology of the corrosion protective layers. Figs. 7 and 8 showthe images obtained from AFM of the inhibitive surfaces in AACmode. Topography (2D and 3D), amplitude, and phase images arepresented in order to get a better understanding of the AFM results.Independently of the immersion time, 3D topographic imagesreveal the presence of clusters on the metal surface; which corre-spond to the dark zones on the phase image. It is noteworthy thatafter 24 h of immersion, there is a higher concentration of clustersand they are distributed more homogeneously. This is especiallyevident in the phase image as the dark areas are much larger thanthose observed at 6 h of immersion, where smaller points of thesame color are observed in a lighter matrix. In accordance withAFM fundamentals (Braga & Ricci, 2004; Yu & Magonov, 2007),phase imaging is very sensitive to surface properties such as stiff-ness, viscoelasticity, and chemical composition. Therefore, the colorscale observed on the phase images indicates the presence of dif-ferent kinds of materials on the metal surface, corresponding lightand dark zones to harder and softer materials, respectively. Thus,according to the inhibition mechanism proposed for this type ofcompounds (Sancristobal, 2009), which was previously mentioned,light areas are attributed to the formation of a thin layer of corro-sion products (iron oxides and hydroxides) and dark clusters areassociated to the presence of activated starch molecules adsorbedon the surface. Furthermore, the small dark spots observed at 6 hof immersion in Fig. 7, are attributed to the insoluble complexformed between the alcoxy groups of AS and iron cations, whichprecipitates sealing pores and/or defects of the corrosion productlayer. This corresponds to the first stage of the formation of a morehomogenous and dense protective film. This in agreement with thecharacteristics signals of starch molecules and corrosion productsobtained from XPS analysis in a previous work (Sancristobal, 2009).

In addition, it should be mentioned that the clusters seem to followa distribution pattern that may be related to the spatial conforma-tion of the starch molecules, in which starch retrogradation may beplaying a role (Thiré, Simao, & Andrade, 2003). Further studies arebeing carried out in order to have evidences of this phenomenon.

566 M. Bello et al. / Carbohydrate Polymers 82 (2010) 561–568

Fig. 7. AFM images of the XC 35 carbon steel surfaces after 6 h of immersion in the presence of 600 mg L−1 of AS.

Fig. 8. AFM images of the XC 35 carbon steel surfaces after 24 h of immersion in the presence of 600 mg L−1 of AS.

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. Conclusions

From this study, it was demonstrated that activated and prege-atinized starches as well as carboxymethylated cassava starchesehave as corrosion inhibitors of carbon steel in solutions that sim-lates tap water. The protection level that they provide dependsn the amount and type of active groups present in the macro-olecules. Even though it was found that the corrosion inhibitor

erformance of carboxymethylated starch (CMS) increases withhe degree of substitution (DS), activated starch (AS) showed bet-er inhibitive properties. This behavior was mainly attributed tohe strong interaction of the active groups present in this moleculeith the metallic surface and to its lower hydrophilicity; variations

f molecular weight also were considered. Moreover, impedanceeasurements carried out in the presence of AS revealed that the

olarization resistances values, which are inversely proportional tohe corrosion rate, increase with immersion time. This fact, whichas explained by the densification of the inhibitive film evidenced

y AFM, is in agreement with the shiny surface of the electrode after4 h of immersion. These results indicate that this biopolymer coulde used as a potential commercial product for open cooling waterystems, encouraging the employment of compounds based on thereen philosophy.

cknowledgments

We gratefully acknowledge funding provided by the Venezuelanational Fund for Research (FONACIT) and Decanato de Inves-

igación y Desarrollo from Universidad Simón Bolívar throughrojects G2005000776 and GID-G02, respectively. We also would

ike to thank K. Contó, MSc, for the preparation of some of thetarch samples employed in this work and J. Sancristobal, Eng, forxperiments concerning CMS0.24.

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